docs: use multiple header levels in virtiofs doc
docs: use multiple header levels in virtiofs doc

The heading overline should only be used for the overall document title,
any subsequent headings should be underline only.

Reviewed-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>
Signed-off-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

olafhering/libvirt,libvirt/libvirt,nertpinx/libvirt,jfehlig/libvirt,olafhering/libvirt,nertpinx/libvirt,jardasgit/libvirt,jardasgit/libvirt,zippy2/libvirt,jardasgit/libvirt,nertpinx/libvirt,jfehlig/libvirt,jfehlig/libvirt,nertpinx/libvirt,libvirt/libvirt,zippy2/libvirt,jardasgit/libvirt,crobinso/libvirt,crobinso/libvirt,crobinso/libvirt,olafhering/libvirt,zippy2/libvirt,zippy2/libvirt,jardasgit/libvirt,libvirt/libvirt,olafhering/libvirt,jfehlig/libvirt,libvirt/libvirt,nertpinx/libvirt,crobinso/libvirt